Our work has been concerned with cancer therapy that targets HER oncoproteins. Our efforts on antibody development have focused on understanding the dominant role of CDR3 heavy chain regions and cross reactivity at the atomic level. We have discovered a new antibody set, 8A4, that is cross-reactive with 2 members of the HER family and interacts even better with dimeric forms of HER2. The antibody disables the malignant phenoytpe of both EGFR transformed, HER2 transformed, and dual EGFR-HER2 heteromeric transformed cells. We will perform affinity maturation and mutation of the 8A4 antibody variable region. Since HER3 and EGFR share similar structural features, we will also examine if we can expand reactivity to HER3 epitopes through light chain mutagenesis to create a tri-reactive MAb. The Fv region of this antibody will be humanized and recombinantly expressed as an intact IgG molecule containing the human Fc region. Such a humanized antibody can be used for future clinical studies in cancers resistant to targeted therapies. HER kinase receptor switching of dimer partners occurs; and cancer resistance can emerge by sequential activities of different HER heteromeric species, HER2-HER2 and then HER2-HER3, for example; and this type of dual or tri-reactive therapeutic will limit that process. In addition, we will combine two subregions of different receptor families. We will use a fragment of human HER2 that is structurally like a CDR and link it to human Fc fragments. Our studies with HER2 receptor peptide mimetics have led to the creation of a receptor derived S22 CDR-like cyclic peptide that binds to EGFR, HER2, and HER3 ectodomains. This molecule can reverse the malignant phenotype in vitro but would require frequent injections in vivo to reduce tumor growth. To extend the therapeutic half-life of S22 CDR, we will engineer a small antibody-like form created by fusing the S22 CDR onto Fc domains. This small novel form will penetrate tumor masses efficiently. Because of its structure it should promote ADCC by preferential interactions with stimulatory Fc Receptors. This novel species should disable EGFR- HER2, HER2-HER3, or EGFR-HER3 complexes and tumor cells expressing them. These efforts are highly innovative and of moderate risk. If successful, new therapeutics for targeted therapy resistant forms of human breast cancer disease will be developed.